Operations of an electric motor configured to drive an electric compressor, which forms a vehicular air conditioning device, are controlled by an electric compressor control system that includes a microcomputer. For example, the electric compressor control system performs control for converting a direct current voltage, which is supplied from a high voltage power supply to drive the electric motor, to an alternating current voltage. In addition, the electric compressor control system performs communication of control signals with an engine control unit (ECU) to control the vehicular air conditioning device, via a communication line.
A vehicular electric system, including the microcomputer and the like of the electric compressor control system, is operated using power supplied from an on-vehicle battery. The voltage supplied from the on-vehicle battery and the voltage of the high voltage power supply for driving the electric motor are significantly different from each other. If the high voltage for driving the electric motor is applied to other electric systems, such as the on-vehicle battery system, this results in failure of the system. Thus, a high voltage system, which is the high voltage power supply system, and a low voltage system, which is the on-vehicle battery system, are insulated from each other.
Here, since the high voltage system is used for driving the electric motor, the voltage and the current thereof fluctuate significantly. This results in a problem in which the high voltage system becomes a source of noise generation in a control circuit.
Thus, in order to reduce a noise current in the control circuit, Patent Document 1 describes a system in which impedances each having the same capacity are inserted between and connected to a frame grounded housing and PN lines configured to allow power to be supplied from a high voltage power supply to an inverter.